/Engine/Source/Runtime/GeometryCore/Public/Intersection/ContainmentQueries3.h
To test if an object is inside an SDF, we first rasterize the object's AABB onto the SDF grid. If all of the AABB cells have negative SDF values, we report that the object is inside. Note this is conservative, not exact  so an object may actually be inside the implicit surface defined by the SDF, but if its AABB is not fully inside then we will report "not inside."
namespace UE { namespace Geometry { template<typename RealType, typename GridType> bool UE::Geometry::IsInside ( const [TTriLinearGridInterpolant](API\Runtime\GeometryCore\Implicit\TTriLinearGridInterpolant)< GridType > & OuterGrid, const [UE::Geometry::TAxisAlignedBox3](API\Runtime\GeometryCore\TAxisAlignedBox3)< RealType > & InnerAABB ) } }